1. Effect of grape proteins on haze formation in wineThe contents of free amino acids, peptides, and proteins in the juices, seeds, and skins from two red and four white grape varieties (Koshu, Semillon, Chardonnay, Riesling, Cabernet Sauvignon, and Muscat Bailey A) were investigated. In the juices, total free amino acids varied with the variety, ranging from 400 to 2230 mg/L at 19oBrix during berry ripening. The soluble protein content ranged from 100 to 500 mg/L, with Semillon grape juice containing the highest amount among six varieties. Less than 1 mg/g of free amino acids was extracted from seeds, and proteins were obtained in the range between 20 and 40 mg/g. The free amino acid content in skins was small, and the amounts of proteins extracted ranged from 1 to 5 mg/g.Since the protein concentrations are low in Cabernet Sauvignon and Muscat Bailey A, which are red wine varieties, it was difficult to obtain proteins from their juice or wine and fractionation by electrophoresis was
… Morenot sufficient. On the other hand, Semillon grape have a high protein content, so in the following experiments, protein were prepared from Semillon grape juice and wines fermented in the same ways as those used to make free-run and red wines.Proteins were fractionated from juice and two wines using salting-out precipitation with ammonium sulfate and HPLC on Superdex 75HR. The proportions of proteins in the protein fractions (lyophilisates) were about 90% in the cases of the juice and free-run wine and about 45% for the wine fermented on skins (the remaining compounds were mostly sugars). The protein fractions were fractionated by gel electrophoresis. The electrophoretic patterns of the juice and free-run wine protein fractions were very similar to each other but very different from that of the fraction from wine fermented on skins.Haze formation was investigated using the protein fractions mentioned above. Proteins and seed phenols were mixed in wine-like model solutions and the mixtures were allowed to stand for 12 hours. They were the centrifuged and the supernatants obtained were analyzed by HPLC. Haze formation differed with the protein judging from HPLC patters (protein peaks) obtained:2. Fractionation and purification of grape polyphenol oxidasesPolyphenoloxidases, by which phenolic compounds were readily bound to proteins to form haze, were fractionated and purified by various column chromatographies and their enzymatic and chemical properties were investigated. The molecular weight of a polyphenoloxidase was about 40 kDa, and its optimum pH was 6.3. Less